For example, a nuclear power plant with a pressurized water reactor (PWR) uses light water as nuclear reactor coolant and neutron moderator, keeps the light water as high-temperature and high-pressure water that is not boiled in the entire reactor core, sends the high-temperature and high-pressure water to a steam generator so as to generate steam by a heat exchange therebetween, and sends the steam to a turbine generator so as to generate electric power.
In such a nuclear power plant, there is a need to periodically inspect various structures in order to ensure enough safety or reliability in the pressurized water reactor. Then, when any problem is found by the respective inspections, the portion causing the problem is repaired. For example, the nuclear reactor vessel body of the pressurized water reactor is provided with a plurality of instrumentation nozzles penetrating a lower mirror. Here, in each instrumentation nozzle, an in-core instrument guide tube is fixed to the upper end inside the reactor and a conduit tube is connected to the lower end outside the reactor. Then, a neutron flux detector capable of measuring a neutron flux is insertable from the instrumentation nozzle by the conduit tube to the reactor core (fuel assembly) through the in-core instrument guide tube.
The instrumentation nozzle is formed in a manner such that the in-core instrument tube formed of nickel base alloy is fitted into the attachment hole of the nuclear reactor vessel body formed of low-alloy steel and is welded by a material formed of nickel base alloy. For this reason, there is a possibility that stress corrosion cracking may be generated in the in-core instrument tube due to the long-time usage, and there is a need to repair the instrumentation nozzle in the event of stress corrosion cracking. As the nozzle repairing method of the related art, for example, a method is disclosed in Patent Literature 1. In a long housing repairing method disclosed in Patent Literature 1, a long housing such as a neutron flux monitor housing fixed and supported to a lower mirror of a nuclear reactor vessel by welding is cut at upper and lower sides of a welding portion, the cut housing is removed, a groove-welding portion of a nozzle of the nuclear reactor vessel is removed along with the left housing, and a groove is restored at a nozzle head. Then, a housing inserted from a through-hole of the nuclear reactor pressure vessel is fixed and supported through a groove-welding portion of a nozzle and an insertion front end is fixed to the housing by welding.